The invention is a facility for removing carbon dioxide (CO2) from the flue gases of coal fired furnaces by absorption in a water spray or dilute aqueous alkaline spray. The use of basic alkaline solutions for the absorption of carbon dioxide has been well established from the earliest chemical studies and has been in use for many years, as in the instance of the Orsat laboratory test apparatus. However, the sequestering of larger quantities of carbon dioxide on a commercial scale from a coal burning furnace by such methods would be financially impractical. This impracticability can be readily appreciated by considering the magnitude of gaseous products produced in the combustion of coal.
If one assumes a completely saturated bituminous chain (hydrogen saturation) comprising methylene intermediate structure and complete combustion to carbon dioxide and water it can be shown that the proportion by weight of carbon reaction product (CO2), relative to reactant carbon (CH2) is about 3 to 1. In other words, the combustion of one ton of coal will produce 3 tons of carbon dioxide. To react 3 tons of carbon dioxide with sodium hydroxide (NaOH) would be financially impractical in considering the general formula, CmHn+(m+n/4)O2→mCO2+n/2H2O, where m is the number of carbon atoms present in the coal, and n is the number of hydrogen atoms. The reactant product would be sodium carbonate (Na2CO3) and as indicated this material imbalance would be further exacerbated by the fact that it would require two sodium atoms to neutralize each carbon atom, and thus prohibitive in cost. The amount of alkaline material available in today's market product flow would not be sufficient in quantity to carry out the neutralization of carbon dioxide in coal fired furnace emissions. The problem is only partly alleviated by the use of calcium hydroxide (CaOH) which reacts on a one for one basis with carbon dioxide to produce calcium carbonate (CaCO3). The primary novelty of this invention is the proposed use of spent electrolyte from electrolytic fuel cells that will be available in sufficient quantities to carry out these reactions. The spent electrolyte is obtained as a by-product of the fuel cell operation discussed in the above Cross References.
It is estimated that the Electrolytic Transportation fuel systems described in the above Cross References will produce about 5 gallons of NaOH solution from the combined initiator reactions (hydrolysis of Na) and calcium moderator reactions (hydrolysis of Ca) from one pound of the alloyed metals (NaCa). This material will be collected at vehicle service stations in tank car quantities to be shipped to coal burning electrical generating facilities for emission control. Noteworthy of this process for the removal of carbon dioxide from coal flue gas is the fact that the sequestered material produced is a saleable value-added commodity which will support the cost of the scrubbing treatment and material recovery operation.
Because of the huge volume of flue gas to be treated as compared with the smaller liquid volume of alkaline spent electrolyte available from the electrolytic fuels program, a unique method of efficiently bringing the two reactants into intimate contact is by use of a liquid to gas ejector system. This effectively improves the material surface contact and mixing characteristics between the two systems which react chemically as a function of the area of the reacting interface and turbulent agitation during the absorption.
Ejectors are used extensively in power generating facilities and have found wide application in the chemical industry. Ejectors may be operated with liquids, gases, or vapors. In the present invention air and water, and in other instances air and spent electrolyte are used as ejector nozzle driving fluids in the formation of absorbent spray in the sequestering process. The momentum exchange energies of these fluid masses passing through the nozzle are used to effect carbon dioxide entrainment by impact with intervening flue gases at the inlet of an axially opposed diffuser section. The resultant force of this impact carries the reacting fluids through the diffuser where they exit as a diffused mixture in a misting zone. At the diffuser exit the gaseous components expand and the liquid components form micron size droplets that have a high surface to volume ratio which increase the absorption rate of carbon dioxide. The small droplets diffuse rapidly in the turbulent system and sequester carbon dioxide in the upward draft of flue gases in the scrubber barrel. At the upper end of the scrubber barrel the diffused droplets are cooled and coalesce into condensed liquid form and pass downward on both sides of the scrubber barrel carrying the sequestered carbon dioxide into the sludge basin where it is pumped to the reclamation facility for further processing into saleable value added substances.
Compared with other gases carbon dioxide is quite soluble in water. The aqueous solutions created in these reactions are acid with a pH of about 4, which are generally considered as being carbonic acid H2CO3 formed by the reaction of the CO2 with the water H2O, although this reaction is only theoretical. However it is generally assumed a small amount of carbon dioxide does react to produce the said carbonic acid H2CO3 and it can be assumed to dissociate to form protons and bicarbonate ion (H2CO3H++HCO3−) which permit the charged reactants to be held in place by electrical charges that are placed on the ejectors and on charge plates immediately adjacent to the induced draft fan.
During an interim period, when electrolytic material is not available, the scrubber can be operated on water alone to produce the said carbonic reaction in the removal of carbon dioxide from flue gases.